Solution and process for etching indium dots



United States Patent SOLUTION AND PROCESS FOR ETCHmG INDIUM DOTS PeterL. Ostapkovich, Poughkeepsie, N.Y., assignor to International BusinessMachines Corporation, New York, N.Y., a corporation of New York NoDrawing. Filed Dec. 30, 1957, Ser. No. 705,826

3 Claims. (Cl. 41-42) This invention relates to a process for cleaningindium dots of etching, preparatory to using the indium dots in themanufacture of semi-conductive devices, typically transistors.

In the manufacture of transistors by the alloy junction method, a mainbody of semi-conductive material, typically germanium or silicon, isalloyed at the interface with a smaller body of a donor or acceptormaterial, commonly called a dot. The usual technique is to place thedots on the surface of the main bodies and fire them in a furnace for aperiod of time and at a temperature sufficient to melt the dot and weldor alloy it at the interface with the underlying body. For themanufacture of PNP transistors, the material commonly used for the dotis indium.

The present invention relates to the cleaning of the dots preparatory tothe firing step of the manufacturing process. The purpose of thecleaning is to remove all foreign materials from the surface of the dot,so as to secure a more uniform alloy junction or weld between the indiumand the germanium.

An object of the present invention is to provide an improved method foretching indium.

Another object of the invention is to provide an improved etchantsolution for use with indium.

The foregoing and other objects of the invention are attained byimmersing the indium clots in a solution consisting essentially of /2 to20 parts hydrofluoric acid, /2 to 20 parts acetic acid and 20 to 100parts water, all parts being by volume. The immersing of the dots ismaintained for a time suflicient to clean the dots, and at a temperaturewhich may be varied inversely with the time, and wherein the time variesmore or less inversely with the concentration of the etching solution.

The two principal ingredients of the novel etching solution arehydrofluoric acid and acetic acid. The process works well at roomtemperature (25 C.). The temperature may be increased up to 45 C. forthe purpose of reducing the etching time. Above that temperature, acidvapor pressures are high and may give trouble, and in any case, theconditions are such as to be highly objectionable to the operators ofthe process.

The time may be varied from a few seconds to several .minutes, dependingupon the acid concentrations used. It is usually best to select acidconcentrations which allow a convenient operating time.

The presently preferred method of carrying out the invention is setforth in the following example:

Example 1 Mix together 5 parts hydrofluoric acid, 5 parts acetic acid in100 parts of water, all parts being by volume. Dip the indium dots to beetched in the acid etching solution so prepared for a period of aboutone minute at room temperature (25 C.). The dots are thereafter rinsedwith water and dried in a current of warm air.

Other illustrative examples of processes embodying the invention are setforth below:

Example 2 Use the same etching solution as in Example 1, but dip thedots in the solution for 7 seconds at a temperature of 45 C.

Example 3 Mix together 20 parts hydrofluoric acid, 20 parts acetic acidand 20 parts water. Dip the indium dots in the acid solution for 10seconds at room temperature.

Example 4 Mix together /2 part hydrofluoric acid, /2 part acetic acidand parts water. Dip the indium dots in the acid solution for 10 minutesat room temperature. This is a presently preferred embodiment of theinvention.

In all of the examples, conventional rinsing and drying processes followthe immersion.

While, in the foregoing examples, equal proportions of hydrofluoric acidand acetic acid have been indicated, and those proportions give the bestresults, nevertheless it is possible to secure a commercially acceptableproduct by varying the proportions of the two acids within the rangefrom about 1 part acetic to about 5 parts hydrofluoric, on the one hand,to about 5 parts acetic to about 1 part hydrofluoric, on the other hand.

In the foregoing examples the hydrofluoric acid is a commerciallyavailable solution of 48% by weight HF, and the acetic acid is acommercially available solution of 99.79% by weight CH COOH, hereinaftertermed glacial acetic acid.

The indium dots etched in accordance with any of the preceding examplesmay then be placed on a germanium body and heated in a furnace for 5 to10 minutes at a temperature of between 560 C. and 610 C. The temperatureselected is a function of the thickness of the germanium body. However,the particular furnace treatment step per se, is no part of the presentinvention.

It has been found that transistors made with indium dots which have beenetched according to the process described above have superior electricalcharacteristics as compared to transistors of the prior art. Inparticular, the cutoff frequencies of the transistors are higher andmore uniform from one transistor to another. Furthermore, the breakdownvoltages across the alloy junctions are also higher and more uniform.

While I have described certain preferred embodiments of my invention,other modifications thereof will readily occur to those skilled in theart, and I therefore intend my invention to be limited only by theappended claims.

I claim:

1. The method of preparing indium dots for use in alloying withgermanium to form semi-conductor devices, comprising immersing said dotsin an aqueous etching solution, the active ingredients of which consistof /2 to 20 parts commercial hydrofluoric acid (about 48% by weight HP)/2 to 20 parts glacial acetic acid 20 to 100 parts water,

present in equal proportions by volume. V

a s3 3. The method of claim 1, in which the active ingredients of saidsolution consist of /2 par-t of said commercial hydrofluoric acid /2part acetic acid 100 parts water said temperature is 25 C. and said timeis 10 minutes.

References Cited in the file of this patent UNITED STATES PATENTSHeidenreich Nov. 25, 1952 Spanos Ian. 24, 1956 Armstrong Sept. 17, 1957Certa Aug. 26, 1958 UNITED STATES PATENT OFFICE CERTIFICATION OFCORRECTION Pa ten no 2 970.0 1 T c L -Q Peter L Ostapiaovicl': It ishereby certified that error a ppears in the above numbered patentreqiiring correction and that the said Letters Patent should read ascorrected below.

Col-mam 1, line 16, for "of etching" read by etching column 3, line 4,after "part" insert Signed and sealed this 13th day of June 1961 SEA L)Attest:

ERNEST W. SWIDER Attesting Officer DAVID L. LADD Commissioner of Patentsof said glacial

1. THE METHOD OF PREPARING INDIUM DOTS FOR USE IN ALLOYING WITHGERMANIUM TO FORM SEMI-CONDUCTOR DEVICES, COMRPISING IMMERSING SAID DOTSIN AN AQUEOUS ETCHING SOLUTION, THE ACTIVE INGREDIENTS OF WHICH CONSISTOF 1/2 TO 20 PARTS COMMERCIAL HYDROFLUORIC ACID (ABOUT 48% BY WEIGHT HF)1/2 TO 20 PARTS GLACIAL ACETIC ACID 20 TO 100 PARTS WATER, ALL PARTSBEING BY VOLUME, WHEREIN THE PROPORTION OF EITHER ACID TO THE OTHER ACIDIS NOT MORE THAN ABOUT FIVE TO ONE, AT A TEMPERATURE OF NOT OVER ABOUT45*C. AND FOR A TIME SUFFICIENT TO CLEAN THE DOTS, BUT NOT OVER ABOUT 10MINUTES.